In domesticated fowl the major histocompatibility complex (MHC) which is associated with the regulation of immune recognition and immune response is called the B system. Resistance to Marek's disease is closely related to the domesticated fowl MHC. Resistance to other diseases, general fitness, and productivity also appear to be influenced to some extent by MHC haplotype.
MHC haplotyping of chickens is presently done by hemagglutination assay which relies on the production of specific antisera. The assay in itself is technically simple. However, the production of the antisera and the interpretation of the assays require a highly trained individual. The MHC haplotypes present in commercial strains of chickens are usually a trade secret known only to individual breeders. Isolation of cloned gene sequences from the B system provides a means of developing alternative methods for MHC haplotyping of birds and for determining the genotype at particular loci within the B system. The interpretation of results is generally simpler and more uniform since typing by restriction fragment length polymorphism patterns is no longer dependent upon alloantisera which often require selective absorptions with blood samples from genetically-defined animals to delineate haplotype specificity.